Hearing-impairment affects about three newborn babies per thousand. These babies must be identified as early as possible so that adequate treatment can be provided while the baby learns to hear and speak. The early detection and treatment of hearing loss helps the hearing-impaired child to communicate effectively, and benefits society since this individual then requires less in the way of support. Governments in Canada, the USA and Europe have therefore instituted programs for universal newborn hearing screening (UNHS). Infants cannot reliably respond to sound—one cannot ask a baby if it can hear a sound. Screening must therefore be performed by non-behavioral/non-subjective measures such as by measuring the response of the ear or the brain to sound. The ear's response can be measured using “otoacoustic emissions” and the brain's response can be tested using the “auditory brainstem response” (ABR). Both tests have their drawbacks. The otoacoustic emissions are fast, but do not check whether the brain is receiving information from the ear. The ABR tests take longer to record than would be optimal for a screening test.
Currently, the click-evoked auditory brainstem response (c-ABR) is the standard test for screening and evaluating infant hearing, but this is often not automatic and is not frequency-specific. While this test will detect gross hearing loss, it may not detect hearing loss at specific frequencies that are important to the development of speech and language, may require a long time in some cases, and may not have high accuracy. Tone-evoked ABRs (t-ABR) can be used to assess frequency-specific thresholds, but this testing procedure takes too long to be used routinely and the results can not be evaluated objectively and automatically by computer (Stapells, 1997). The use of otoacoustic emissions (OAEs) is popular for quick screening of normal auditory function, but will only detect peripheral loss and, while OAE methods can detect hearing impairment, these techniques cannot be used to determine the actual extent of the hearing loss. Frequency-specific audiometric techniques that are rapid and accurate are therefore important for detecting auditory thresholds and fitting hearing aids in infants, or other patients, who are unable to easily provide reliable indication of their hearing abilities.
There are two main types of tests that may be implemented by an audiologist: screening tests and threshold tests. These tests can be carried out for either frequency-specific stimuli, or for non-frequency specific stimuli. For example, current UNHS tests are done using non-frequency-specific click stimuli which contain energy at many frequencies. These tests indicate whether or not an infant has a minimal acceptable level of hearing and usually provide a simple pass/fail result. In the case where an individual fails a screening test (hearing thresholds are elevated), a threshold test can provide a further assessment of auditory abilities. In a threshold test, an individual's hearing is tested at successive intensity levels in order to determine the thresholds of a patient (i.e., the minimum level at which a patient can hear a sound). There is a need for objective frequency-specific threshold tests that can be performed relatively quickly since conventional objective hearing threshold tests require on the order of 30-40 minutes for obtaining a hearing threshold at multiple frequencies. This amount of time prevents the test from becoming clinically feasible in some cases because, for example, a sleeping infant may wake up and start crying, which makes testing impossible.